1. Field of the Invention
The present invention relates to an abnormality determining apparatus and an abnormality determining method for determining the presence or absence of an abnormality in a detection subject based on a result obtained by an information obtaining device that obtains information of the detection subject.
2. Discussion of the Background
An abnormality determining apparatus that determines the presence or absence of an abnormality in a detection subject has been used. For example, a thermal test device used in an electric control circuit is an example of an abnormality determining apparatus that determines an abnormality in an electronic product acting as a detection subject when the electronic product consumes electric power exceeding a threshold value. This thermal test device determines the presence of an abnormality based on one type of parameter such as electric power consumption. However, in various types of devices on the market, it may be difficult to determine the presence of an abnormality based on only one type of parameter, and it may be necessary to take an interrelation among a plurality of parameters into consideration to detect an abnormality.
For example, in an image forming apparatus a decrease of density of an output image may occur. Even if an image forming apparatus has no abnormality, the decrease of density of an output image may similarly occur. Specifically, when solid images are continuously output while consuming a large amount of toner and a developing device cannot be replenished with a sufficient amount of toner, the decrease of density of an output image typically occurs even if the apparatus has no abnormality. In this case, it cannot be determined whether such a density decrease is caused by an abnormality of the apparatus based on only one parameter such as density of an output image. It is necessary to determine the presence of an abnormality based on an interrelation among a plurality of parameters, such as a number of continuous prints, and an output image pixel of each print.
A Mahalanobis Taguchi System (MTS), described in the Japanese Standards Association publication “Technical Developments in the MT System” by Genichi TAGUCHI, is known as a method of determining an abnormality. This MTS method measures the degree of normality in the state of an object in the following manner. First, a plurality of combined normal values relating to a group of information including a plurality of types of information indicating the state of a detection subject are obtained, and a normal group data set is constructed therefrom. To take a medical checkup as an example, first combined normal values including the gender, various blood test results, height, weight, and so on of a healthy person are obtained in advance from a plurality of healthy people, and a normal group data set is constructed therefrom. Next, a multidimensional space is constructed based on the normal group data set. The Mahalanobis distance, which indicates the position of the grouped information obtained from the detection subject in this multidimensional space, is then determined, and an evaluation is performed to determine the degree of similarity between the grouped information for the detection subject and the normal group data set. With this MTS method, the degree of normality of the detection subject can be determined comprehensively based on the interrelation among a plurality of parameters.
The present inventors are currently developing a novel abnormality determining apparatus that determines the presence of various types of abnormalities in an image forming apparatus by using the MTS method. However, the following problems still need to be addressed.
The first problem is that an operation for constructing a normal group data set is complicated. There are two main methods of constructing a normal group data set of an image forming apparatus. In the first method, various types of parameters are obtained while making test runs of a normal standard machine of an image forming apparatus, and a normal group data set is constructed based on the obtained results. Such a normal group data set is used as a common normal group data set for each image forming apparatus shipped from a factory. In the second method, various types of parameters are obtained while making test runs of each of image forming apparatuses in a normal state before factory shipment. Then, a unique normal group data set is individually constructed based on the obtained results for each image forming apparatus.
In the first method, an operation for constructing a normal group data set can be efficiently performed only one time. However, because variations in a component size and in accuracy of attachments of components among image forming apparatuses are not reflected in a normal group data set, accuracy of abnormality determination is degraded. For this reason, the second method is preferably used, which can reflect such variations in a normal group data set. However, in the second method, production costs significantly increase due to complicated operations for obtaining various types of parameters while making test runs of each product at the factory before shipment.
The second problem is that abnormalities are not always detected at a timing which is appropriate for each user. Specifically, depending on the type of fault, each user has a widely differing perception thereof. For example, one user may suspect a fault in a sheet feeding system even if a sheet jam occurs rarely, whereas another user may not suspect a fault in a sheet feeding system even if sheet jams occur comparatively frequently. In the case of the latter user, if the user is notified of an occurrence of a fault in a sheet feeding system when an abnormality rarely occurs in the sheet feeding system, the user feels inconvenienced.
These problems relating to an abnormality determining apparatus are described on the assumption that an image forming apparatus is a detection subject. However, similar problems may occur when determining the presence or absence of abnormality in a detection subject other than the image forming apparatus.